A stroke-enlarging type brake booster mentioned herein means one wherein a power piston, which is so disposed in a booster casing as to divide the interior of the same into two chambers and actuated by a pressure difference between the two chambers, is separated from a controlling mechanism for controlling the pressure difference upon operation of an input member for being relatively movable to the controlling mechanism. This stroke-enlarging type brake booster is featured in obtaining a larger output stroke than the input stroke, while in ordinary brake boosters the output stroke can never be larger than the input stroke.
The stroke-enlarging type brake booster is still disadvantageous, when there is a certain limit in a assisting or promoting force, i.e., a force produced by the difference of pressure on opposite sides of the power piston, in that an attempt of getting a larger output than a critical output obtained at the upper limit of the assisting force results in a temporary rapid increasing of the pedal stroke (an input stroke) without a resultant increase of the braking force.
Two of the applicants of this invention earlier filed a patent application for a development capable of eliminating this disadvantage, in the United States, Serial Number of which is 71,489 (now, U.S. Pat. No. 4,287,811). In the brake booster of this type two of the undermentioned members are disposed on either one of the power piston and a control piston, which is a part of the controlling mechanism, (a) a locking member movably retained substantially perpendicular to the direction of the movement of the two pistons, operated between the forwardly advanced position and the backwardly retracted position owing to the pressure difference between the two chambers of the power piston, and engaged with an engaging portion formed on the other of the two pistons so as to block the relative movement of both pistons, and (b) an elasticity device constantly retaining the locking member at the backwardly retracted position, and allowing the locking member to advance, when the pressure difference between the two chambers exceeds a predetermined value, to the forwardly advanced position to engage the engaging portion.
For movably retaining the locking member, however, either one of the power piston and the control piston must be made to be considerably large, and chambers on opposite sides of the locking member and the chambers on opposite sides of the power piston must be communicated with passages purposely arranged, which inevitably makes the structure of the locking mechanism complicated. As the locking member is engaged with or released from the engaging portion due to the comparative magnitude between the force created from the pressure difference between the two chambers thereof and the elastic force of the elasticity device, there is a disadvantageous tendency of delaying the operation.